1. Field of the Invention
This invention relates generally to the transfer of data in a serial manner. In particular, this invention relates to the transfer of data between a dispatcher and a receptor in such a manner as to be completely independent of any clocking scheme.
2. Description of the Prior Art
Both method and apparatus for the serial transfer of data in a rate independent fashion have been disclosed in U.S. application Ser. No. 534,972 entitled "Rate Independent Signalling Means", filed on Dec. 20, 1974 by Joseph J. Eachus now U.S. Pat. No. 4,002,833. This particular application disclosed a manner of transmitting information from a dispatcher to a receptor without benefit of a specified rate of data transmittal. This was accomplished by coding signal transitions between four signal states that were being transmitted between the data dispatcher and the data receptor.
The transitions between the four signal states occur by way of signal state changes to two bi-level signals generated by the data dispatcher. The data dispatcher is operative to change the signal state of either or both bi-level signals depending on which of three discrete units of information is to be thereby encoded. In the preferred embodiment, a change in signal state of only one bi-level signal is indicative of a data binary one whereas a change in signal state of the other bi-level signal is indicative of a data binary zero. A simultaneous change in signal state of both bi-level signals indicates an end of data.
The above encoding of data was achieved by internal logic within the data dispatcher which was capable of timely executing the various signal transitions. In this regard, the internal logic is premised on a clocking signal which provides the requisite timing necessary to produce the variously occurring signal transitions. This internal clocking within the data dispatcher is preferably at a given rate even though such rate need not be known to the data receptor.
It is therefore to be appreciated that the aforementioned system is not completely clockless insofar as the encoding of the data by the data dispatcher. It is furthermore to be appreciated that the receptor must be capable of receiving the data at the rate of transmittal dictated by the dispatcher. In this regard, there is no assurance that the data receptor will be capable of receiving the data at the rate in which it is being transmitted by the dispatcher. Such a system allows for the possibility of a rate of transmittal by the dispatcher which will be high enough to override the response characteristics of the receptor. Such a rate of transmittal would of course result in a loss of data by the receptor.